1. Field of the Invention
The present invention relates to the field of transistors and more particularly to average and high power transistors used in switching, that is to say capable of going very quickly over from the conducting state to the disabled state.
2. Description of the Prior Art
These transistors generally have interdigitated emitter and base structures, that is to say that the emitter and base zones comprise at least partially portions in the form of imbricated strips.
FIG. 1 shows a partial perspective view in cross section of a digitated emitter transistor, this view being schematical. This transistor comprises N.sup.+ and N type collector layers 1 and 2 and a P type base layer 3, in which are formed generally by diffusion N.sup.+ type emitter zones 4 having the form of elongate fingers. Between the emitter fingers 4 there may possibly be provided base fingers 5 formed of P type zones with a doping level higher than the doping level of layer 3 so as to promote the ohmicity of the contacts. Above the emitter diffusions 4 are provided emitter metalizations 6 and, between these emitter metalizations, are provided base metalizations 7. The lower face is coated with a collector metalization 8. The upper face of the wafer, outside the contact zones with the metalizations, is protected by a silicon oxide layer 9 (SiO2).
FIG. 2 shows a view in enlarged and schematic section of the structure of an emitter finger. There can be seen therein the base layer 3, the emitter zone 4, the emitter metalization 6 and the oxide layer 9. At the moment when it is desired to interrupt the passage of the current through the transistor, it is the lateral parts of the emitter zone which will be affected first by the currents from the adjacent base electrodes 7 and the current lines are concentrated in the central part of the emitter zone. This phenomenon of concentration of the current lines on opening which slows down switching over to the disabled state of a transistor has been studied by numerous authors under the term of focussing of the current lines.
Different remedies have been proposed for palliating this drawback.
A first remedy, illustrated in FIG. 3, consists quite simply in removing the central part of the emitter finger and in coating it with an oxide layer 10 before providing the metalization 6. Each emitter finger is then divided into two distinct fingers 11 and 12 connected together by the metalization 6, this metalization having substantially the same width and the same thickness as that shown in FIG. 2, the section of this metalization being determined with respect to the current which it is desired to pass through the emitter and so as to obtain a substantially equipotential area over the whole surface of the emitter zone. A drawback of the structure of FIG. 3 is that if the oxide layer 10 has a defect (a hole), the metalization 6 will short-circuit the base and emitter zones.
To avoid this drawback, the prior art has also proposed the structure illustrated in FIG. 4 in which each emitter finger is also divided into two strips 11 and 12, but where a diffusion 13 of doping atoms of the same type as those of zones 11 and 12 is provided in the central part and where the metalization 6 is laid over the whole emitter zone including the central part without insulation. Thus, possible base/emiter short-circuits are avoided which may result from structural defects in the oxide layer 10 of FIG. 3. Nevertheless, the structure of FIG. 4 has the obvious disadvantage of being less efficient than the structure of FIG. 3 because, in spite of everything, there occurs an injection at the level of the central part 13 of the emitter.
It will be further noted that, in the case of FIGS. 3 and 4, an injection occurs in the neighborhood of the central zone from the lateral parts 14 and 15 of the strip shaped emitter zones 11 and 12.